The present invention relates to a method of making a silicon pressure sensor that will withstand hostile environments that normally attack standard IC metalization.
A typical silicon pressure sensor has a silicon diaphragm onto which piezo resistive regions are placed to sense movement in the diaphragm. Metalization needs to connect to the resistors so they can be connected to a monitoring circuit. Pressure sensors are often used in corrosive applications, and thus the metalization is typically exposed to the gas or liquid whose pressure is being measured.
One of the challenges in silicon pressure sensor technology is to develop a metalization that is resistant to corrosion. Most applications of pressure sensors require, ultimately, the exposure of the top surface of the sensor to liquids and gases that may tend to corrode metal. The metal of choice for most ICs and Sensors has usually been aluminum or an aluminum composite (Al/Al+1% Si/Al+1% Cu). FIG. 1 illustrates the cross-section of this standard approach.
Exposure to even mild acids rapidly devolves this metalization. To overcome this, a number of approaches have been attempted. These include simply overcoating the aluminum with a noble metal such as gold or gold with an adhesion layer where the aluminum would normally be exposed to the hostile environment. Other approaches have included using Chrome/Gold or Titanium/tungsten/Gold (TiW/Au) directly on silicon contacts. An example of this cross-section is depicted in FIG. 2.
The Chrome--Gold approach has major drawbacks in that there is a tendency to have gold migrate through the chrome, resulting in "purple-plague" and ultimately a degraded performance. Without a good inter-layer, TiW does not always form a good ohmic contact, rendering the Chrome--Gold approach ineffective.